There are lots of possible reasons we haven't (to our knowledge) encountered aliens (as in, an intelligent space-faring race), but these two stand out in my mind as the biggest and most obvious explanations:
1)Space is big. Like, it's really, really big. Assuming that you can't actually break the speed of light barrier, it's more than possible that in the span of one trip to another planet, their civilization has gone extinct. In fact, it's entirely possible that the last time aliens came here, the planet just had single cell lifeforms. And it's perfectly possible they'll pop by again for another visit and...to late! We're extinct. Or they'll finally get here in six million years when we evolved into different species twenty times over.
2)They popped in to take a look and didn't find us very interesting. I mean, think about it. If you're part of an interstellar, space-faring civilization, what are you going to be so amazed about here? How many trailer parks have you passed by while barreling down the freeway at 85mph? How many times have you said to yourself, "Oh look, a trailer park. I'm going to go check them out."